I like it a lot. I haven't used them on-board, so I can speak for that. As others have said, there isn't a lot of bass but it does sound good.
The STX definately has enough power to drive the DT880 250's.
I was going to buy a really nice amp for them, but after reading that if you use an external amp you can't get positioning information properly for games, I opt'd to just use the on-board amp. I am likely going to replace the OpAmps and try that out.
It is a lot better than what I had before, and very happy with it. Still trying to get the best settings for the setup. Tried a few things and hard to come up with what is ideal.
I was thinking the same thing when I first got my Beyer 990 600ohm. Check out my custom EQ generator for Xonar Audio Center to help boost the low end.